1. Technical Field of the Invention
The present invention relates to a Faraday rotator in which a Faraday element and an external magnetic rield applying means are arranged such that the magnetization direction of the Faraday element is tilted with respect to a light ray direction, and particularly to a Faraday rotator capable of reducing the amount of the temperature-dependent change in Faraday rotation angle by making use of the temperature dependence on an angle .alpha. between the magnetization direction of the Faraday element and the light ray direction. Such a Faraday rotator is useful for various optical devices utilizing the Faraday effect, such as an optical attenuator, and an optical isolator.
2. Related Art
Optical communication systems require an optical isolator for allowing light rays to pass therethrough only in one direction, an optical attenuator for controlling the quantity of light rays passing therethrough, etc., and a Faraday rotator for rotating the polarization plane of light rays passing therethrough is incorporated in the optical isolator, optical annenuator, etc. The Faraday rotator is also used for other optical devices such as an optical switch, optical circulator, optical filter, and optical equalizer.
An optical isolator has a configuration, for example, shown in FIGS. 21A and 21B in which a 45.degree. Faraday rotator 3 is inserted between a polarizer 1 and an analyzer 2 which are arranged such that the polarization planes of light rays passing through the polarizer 1 and analyzer 2 intersect one another at 45.degree.. The Faraday rotator 3 includes a Faraday element composed of a magnetooptic crystal in combination with a permanent magnet as an external magnetic field applying means. An external magnetic field is applied to the Faraday element by the permanent magnet in such a manner as to correspond to a light ray direction, to realize a magnetic saturation state of the magnetooptic crystal. The magnetooptic crystal is designed to have a thickness allowing the polarization plane of light rays passing therethrough to be rotated 45.degree. in the above magnetic saturation state. When light rays are allowed to pass through the optical isolator in the forward direction, the light rays having passed through the polarizer 1 pass through the analyzer 2 almost with no loss (see FIG. 21A). On the contrary, when light rays are allowed to pass through the optical isolator in the reverse direction, the light rays having passed through the analyzer 2 cannot pass through the polarizer 1 because the polarization plane of the light rays having passed through the Faraday rotator 3 are rendered perpendicular to the polarizer 1 (see FIG. 21B). This optical isolator is of a polarization-dependent type; however, there is also known a polarization-independent type (see Japanese Patent Application No. Sho 56-148290).
One example of a prior art optical attenuator is shown in FIGS. 2A and 2B. As shown in FIG. 2A, a polarizer 14 composed of a wedge-shaped birefringent crystal (for example, rutile), a Faraday rotator 15, and an analyzer 16 composed of a wedge-shaped birefringent crystal 16 are arranged on the optical axis in this order between an input fiber 12 having a collimate lens 10 and an output fiber 13 having a collimate lens 11 (see Japanese Patent Application No. Hei 4-205044). The Faraday rotator 15 includes, as shown in FIG. 2B, a Faraday element (magnetooptic crystal) 17 in combination with a permanent magnet 18 and an electromagnet 19 for applying magnetic fields to the Faraday element 17 in two directions which are 90.degree. offset from each other. The magnetization direction of the Faraday element 17 is matched with the direction of a synthetic magnetic field of a specific magnetic field applied by the permanent magnet 18 and a variable magnetic field applied by the electromagnet 19. Therefore, the Faraday rotation angle is variable.
For example, when the polarizer 14 and the analyzer 16 are arranged such that the optical axes of both the birefringent crystals thereof are rendered parallel to each other, the optical attenuator operates as follows. Light rays having gone out of the input fiber 12 are converted into a collimated light beam through the first lens 10 and are separated into an ordinary light ray o and an extraordinary light ray e through the polarizer 14. The polarization direction of the ordinary light ray o is perpendicular to that of the extraordinary light ray e. When the light rays o and e pass through the Faraday rotator 15, the polarization direction of each of the light rays o and e is rotated depending on the magnitude of a component of the magnetization of the Faraday element 17 in the direction parallel to the optical axis. The light rays o and e are then separated, through the analyzer 16, into an ordinary light ray o.sub.1 and an extraordinary light ray e.sub.1, and an ordinary light ray o.sub.2 and an extraordinary light ray e.sub.2, respectively. As shown by solid lines in FIG. 2A, the ordinary light ray o.sub.1 and extraordinary light ray e.sub.2 outgoing from the analyzer 16 are parallel to each other, and are coupled to the output fiber 13 through the second lens 11. Meanwhile, as shown by broken lines in FIG. 2A, the extraordinary light ray e.sub.1 and ordinary light ray o.sub.2 outgoing from the analyzer 16 are not parallel to each other but spread outwardly, and are not coupled to the output fiber 13 through the second lens 11.
When the magnetic field applied to the Faraday element 17 by the electromagnet 19 comes into zero, that is, when the magnetization direction of the Faraday element 17 is rendered parallel to the optical axis, the Faraday rotation angle of the Faraday element 17 becomes 90.degree.. At this time, the ordinary light ray o having gone out of the polarizer 14 goes out of the analyzer 16 as the extraordinary light ray e.sub.1. The extraordinary light ray e having gone out of the polarizer 14 goes out of the analyzer 16 as the ordinary light ray o.sub.2. The light rays e.sub.1 and o.sub.2 are spread outwardly, and are not coupled to the output fiber 13 through the second lens 11. On the contrary, when the magnetic field applied to the Faraday element 17 by the electromagnet 19 becomes sufficiently large, the Faraday rotation angle of the Faraday element 17 comes closer to 0.degree.. At this time, almost all of the ordinary light ray o having gone out of the polarizer 14 goes out of the analyzer 16 as the ordinary light ray o.sub.1, and almost all of the extraordinary light ray e having gone out of the polarizer 14 goes out of the analyzer 16 as the extraordinary light ray e.sub.2. The light rays o.sub.1 and e.sub.2 are parallel to each other, and are all coupled to the output fiber 13 through the second lens 11. The magnetization of the Faraday element 17 is thus rotated depending on the strength of the magnetic field applied to the Faraday element 17 by the electromagnet 19, to change the Faraday rotation angle of the Faraday element 17 in a range of about 90 to about 0.degree., thereby making variable the quantity of the light rays coupled to the output fiber 13 in accordance with the amount of the change in Faraday rotation angle. In this way, the above configuration including the Faraday rotator 15 functions as an optical attenuator.
It should be noted that if the polarizer 14 and the analyzer 16 are arranged such that the optical axes of both the birefringent crystals thereof are perpendicular to each other, the optical attenuator operates in accordance with the manner reversed to that described above. That is to say, when the Faraday rotation angle of the Faraday element 17 becomes 90.degree., the quantity of light rays passing through the optical attenuator is maximized, while when the Faraday rotation angle of the Faraday element 17 becomes zero, the quantity of light rays passing through the optical attenuator is minimized.
As the Faraday element to be incorporated in the Faraday rotator, there has been, in recent years, used a Bi (bismuth) substitution rare earth element-iron garnet single crystal film (LPE film) which has been mainly manufactured by the LPE (Liquid Phase Epitaxial) Method. The reason for this is that the LPE film has a large advantage that the Faraday rotation coefficient is larger than that of a YIG (yttrium-iron garnet) single crystal by the effect of addition of Bi.
The Bi substitution rare earth element-iron garnet single crystal, however, has a disadvantage in that the temperature dependence on the Faraday rotation angle is large. This causes a problem in increasing the temperature dependence on the Faraday rotator, thereby making large the temperature characteristic of an optical device manufactured using the Faraday rotator, such as an optical isolator or optical attenuator.
To improve the above-described temperature characteristic of the Faraday rotator, there have been typically proposed the following three methods:
(1) to improve the physical properties of the conventional crystal by replacing it with a crystal having a special composition (Japanese Patent Application No. Sho 60-243217); PA1 (2) to improve the temperature dependence on the Faraday rotator by using two garnet crystals as a magnetooptic element for canceling the temperature dependence on the Faraday rotation angle of one garnet crystal by that of the other garnet crystal (Japanese Patent Application Nos. Sho 60-134372 and Hei 2-180757); and PA1 (3) to improve the temperature characteristic of an optical device using a Faraday rotator, a polarizer and an analyzer by optimally arranging the polarizer and the analyzer (Japanese Patent Application No. Hei 8-45231).
The above proposed methods, however, have the following problems:
In the method (1), the temperature dependence on the Faraday rotation angle is reduced by adding Tb to the Bi substitution rare earth element-iron garnet crystal as the magnetooptic element. However, an optical isolator described in the embodiment is configured such that the thickness of a magnetooptic element of the optical isolator at a wavelength of 1.5 .mu.m at which the temperature dependence is minimized becomes about 1,700 .mu.m. This thickness is excessively large in consideration of the fact that the critical film thickness of a crystal allowed to be grown by the LPE method with its quality highly kept is about 500 .mu.m.
In the method (2), since two different garnet crystals must be manufactured, the manufacturing cost becomes high.
In the method (3), an optical attenuator is realized by making use of the fact that the maximum light attenuation point (amount) in a state that the polarization plane of light rays is perpendicular to the polarizer is sensitive to the angle of the polarization plane, that is, the Faraday rotation angle, while the maximum light transmission point (amount) in a state that the polarization plane of light rays is parallel to the analyzer is insensitive to the Faraday rotation angle. That is to say, in the optical attenuator, the temperature dependence on each of the maximum amount of light attenuation and insertion loss (maximum amount of light transmission) is made smaller by making the polarization plane of light rays parallel to the analyzer when the Faraday rotation angle is maximized (at this time, the absolute value of the amount of the change in Faraday rotation angle is maximized); and making the polarization plane of light rays perpendicular to the analyzer when the Faraday rotation angle is minimized (at this time, the absolute of the amount of the change in the Faraday rotation angle is minimized). However, since the maximum light attenuation point is sensitive to the Faraday rotation angle, the temperature dependence on the Faraday rotation angle must be made very small at the maximum light attenuation point, but the reduction in temperature dependence has a limitation because the Faraday rotator essentially has the temperature dependence.